The adult central nervous system has been shown to be capable of significant plasticity. Electrical stimulation of the somatosensory pathway has increased cortical excitability in both normal subjects (Ridding & Rothwell, 1999; Ridding et al. 2000; McKay et al. 2002; Khaslavskaia et al. 2002) and patients with brain lesions (Fraser et al. 2002). However, a number of stimulation patterns have been used and the relationship between these and the extent of cortical change is unknown. There have been no studies of the differential effects of voluntary and different patterns of electrically stimulated muscle activity on cortical excitability. We investigated this in normal subjects.
With local ethical approval the wrist extensor muscles of 5 healthy subjects (22-57 years, 3 females) were activated for 60 min on 5 occasions at least one week apart using different patterns of activity. Passive stimulation was performed at 5 Hz, 300µs (Fraser et al. 2002); 25 Hz, 300 µs (Powell et al. 1999) and 10 Hz train of 1 ms pulses for 500 ms s-1 (Ridding et al. 2000). EMG triggered stimulation was performed at 25 Hz, 300 µs (Cauraugh et al. 2000) and voluntary exercise for 5 s every 25 s. Motor evoked potentials (MEPs) of the same muscles were elicited by transcranial magnetic stimulation (1.2 times threshold) at 10 min intervals before, during and for 20 min after muscle activity. The amplitude of 10 averaged MEPs was measured. The mean MEP amplitude before stimulation was 1.35 mV (S.D. 0.71 mV). None of the interventions caused a significant change in the MEP amplitude at any time and there were no significant differences between them.
These results indicate that a single 60 min episode of voluntary or stimulated activity of the wrist extensors has no effect on cortical excitability in normal adult brains regardless of the stimulation pattern. This is in contrast to the reported increase in cortical excitability in the pharyngeal muscles after only 10 min (Fraser et al. 2002). It remains to be seen whether brains recovering from neurological insults respond differently.
Isaac Sorinola is sponsored by the Association of Commonwealth Universities PhD studentship